It is known to construct charge redistribution analog-to-digital converters (ADC's) using integrated circuit technology. These integrated ADC's trap a charge proportional to the analog voltage to be measured, and then gauge the amount of charge trapped by testing it against differently-sized capacitors in an array. They tend to remain linear over their operating temperature range because the capacitance of capacitors made using integrated circuit technology usually does not to vary much in response to temperature changes. In addition, because the capacitors are in parallel, only differences in their temperature coefficients will affect linearity, and these differences can be significantly reduced by closely integrating the arrays.
These performance characteristics are not usually available immediately after supplying power to the part. Typically, the part must be allowed to settle before it reaches normal operating conditions, which can be defined as a state where the device operates consistently within a certain range of performance parameters, such as accuracy, linearity, or offset ranges. Before the part reaches this condition, its performance is affected by transient effects within the part, such as the charging of capacitors, or possibly the warming of circuit components.
It is known to provide calibration circuitry for these converters to compensate for manufacturing tolerances, drifts, or the like. For example, the calibration circuitry can include a switched array of capacitors that acts like a variable capacitor. This type of array can be connected in parallel with a capacitor in the ADC to allow adjustment of the total capacitor capacitance until it reaches a desired value. Typically, initiating a calibration operation involves asserting a signal on a calibration control pin, or providing a calibration command to a control register within the part via the part's bus interface, and calibration is usually not initiated until the part has been powered up for a certain interval, to allow the converter to reach its normal operating conditions.
Analog-to-digital converters can also include "shutdown" circuitry, which reduces power consumption to a minimum. When the converter is in shutdown mode, the analog circuitry in the converter receives only very small leakage currents. Upon returning from shutdown mode to normal operation, the ADC circuitry should generally be allowed to again reach normal operating conditions before relying on conversion values, to ensure accurate results.
Present converters usually require that the system designer provide circuitry and/or software that initiates power-up calibration. In addition, if the user intends to immediately place the part in shutdown mode after power-up, the user must typically first initiate calibration of the part and then provide a shutdown command to the part.